ORA-12704: character set mismatch

you should do

select COALESCE (EMAIL, n'NO EMAIL') from crmuser.accounts

to convert the literal to NVARCHAR.

eg http://sqlfiddle.com/#!4/73929/1 vs http://sqlfiddle.com/#!4/73929/2

This generic fix works with columns defined as either VARCHAR2 or NVARCHAR2:

select COALESCE (EMAIL, N'' || 'NO EMAIL') from crmuser.accounts

Just add N'' || before your non-Unicode string constant.

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